1. Field of the Invention
The present invention relates to an imaging device to be used for taking an image of an object, and a photodetector for use in imaging.
2. Description of the Related Art
In recent years, imaging devices such as CCDs and CMOS sensors are used widely in small-sized electronic appliances such as digital cameras and mobile phones. The structure of such an imaging device is disclosed in, for example, Optical Alliance, 12, No. 5 (2001) pp. 1–14.
Hereinafter, referring to FIGS. 8A and 8B, main component elements of conventional imaging device will be described. FIGS. 8A and 8B show cross-sectional structures of an imaging device and a photodetector for use in imaging, respectively.
As shown in FIG. 8A, light (e.g., natural light) which is reflected by an object 1 forms an image 3 on a light-receiving surface of a photodetector 4 (e.g., a CCD or a CMOS sensor), through a lens system 2. Although a commonly-used type of lens system 2 is a combination of a plurality of lenses which are arranged along an optical axis for realizing together a desired optical performance, it is assumed for simplicity herein that the lens system 2 is composed only of one lens.
The photodetector 4 comprises a substrate 5 having a two-dimensional array of photosensitive portions 6 arranged along a light-receiving surface, and a transparent layer 7 which is deposited on the light-receiving surface. A light-shielding mask 8 is embedded within the transparent layer 7, with color filters 9, a transparent layer 10, and microlenses 11 being stacked upon the light-shielding mask 8. Each unit structure (referred to as a “photodetection cell”) from the photosensitive portion 6 to the microlens 11 detects light corresponding to a pixel of a single color. With respect to the three primaries of red (R), green (G), and blue (B), by combining signals obtained from the photodetection cells of the pixels corresponding to the three respective colors, a “color image” can be reproduced.
The photosensitive portions 6 are formed at nodes of an orthogonal lattice within the photodetection surface, the photosensitive portions 6 being insulated and spaced apart from one another. In these interspaces, interconnects for transferring detected signals are provided. In the case of a CCD, the interspaces will be used for vertical transfer and horizontal transfer. The light-shielding mask 8 is formed so as to overlie each interspace between the photosensitive portions 6, such that each single photosensitive portion 6 is not covered by the light-shielding mask 8. The center of each color filter 9 and the center of each microlens 11 are aligned with the center of the corresponding photosensitive portion 6, and each opening in the light-shielding mask 8 is covered by the corresponding color filter 9.
All photosensitive portions 6 in the two-dimensional array are of the same structure, whereas the color filters 9 fall into three types: red pass filters, green pass filters, and blue pass filters. The three types of color filters 9 have respective light-transmitting characteristics as follows: the red pass filters cut off light of any wavelength other than red; the green pass filters cut off light of any wavelength other than green; and the blue pass filters cut off light of any wavelength other than blue.
Every four photosensitive portions 6, i.e., one for red detection, one for green detection, one for blue detection, and one for brightness detection, compose a set. The four photosensitive portions 6 may be arranged in a clockwise order on a square, with red pass, green pass, blue pass, and green pass color filters 9 being employed respectively in combination therewith. Such an arrangement of color filters 9 is known as the Bayer pattern. The reason for using a green pass color filter 9 for brightness detection is that the human eyes are most sensitive to green. As shown in FIG. 8B, each microlens 11 refracts incident light which enters with an offset from the center axis of the microlens 11 (e.g., a ray 12), and deflects the light (which would otherwise have been blocked by the light-shielding mask 8) toward the opening, thus leading the light to the photosensitive portion 6.
FIG. 9A is an upper plan view showing a part of the conventional photodetector 4, where pixels 16 are arranged at nodes of an orthogonal lattice with equal distances (pitch Λ).
Resolution and brightness are among fundamental elements which govern the performance of an imaging device. Resolution is generally related to the total number of photosensitive portions 6 (number of pixels). However, under a fixed focal length f of the lens system 2, resolution is related to the size or density of the photosensitive portions 6.
On the other hand, brightness is related to the amount of light detected by each single photosensitive portion 6. As the size of the photosensitive portions 6 decreases, resolution is improved, but brightness is degraded because the amount of detected light decreases. Conversely, as the size of the photosensitive portions 6 increases, brightness is improved, but resolution is degraded.
Brightness is also related to the focal length f and the aperture (diameter D) of a lens. The value of f/D (=focal length/aperture) is referred to as an “F value”. As the F value decreases, the resultant image becomes brighter. The reason is that, as D increases, the image becomes brighter because of being able to take in light through a broader area; as f decreases, the image becomes brighter because the size of the image is decreased, since the light intensity of an image generally increases in inverse proportion to the image size.
Resolution is also related to the optical performance of the lens system 2. If the lens system 2 has a high optical performance so that the image 3 is being reproduced all the way up to high frequency patterns (i.e., patterns with fine pitches), resolution is also improved. On the other hand, if the lens system 2 has a poor optical performance so that the image 3 can only be reproduced with respect to low frequency patterns, resolution is degraded. The microlenses 11 provides some contribution to brightness improvement because the microlenses 11 function to lead light which would otherwise have been blocked by the light-shielding mask 8 to the photosensitive portions 6.
The above-described conventional imaging device and photodetector for use in imaging have the following problems. As shown in FIG. 8B, for example, a ray 12′ which is oblique with respect to the center axis of the microlens 11 is blocked by the light-shielding mask 8 because a lens has such general characteristics that it allows any light traveling through its principal point to pass straight through. Therefore, the effect of the microlenses 11 is not sufficient, and the detection efficiency will always be worse for any oblique ray. For example, at the center axis of the lens system 2, i.e., in a region away from the center of the photodetector 4, the rays are greatly tilted with respect to a normal of the detection surface, so that the brightness is not sufficient as compared to the neighborhood of the center of the photodetector 4; thus, a correction process will have to be performed during image processing. This problem associated with oblique rays requires that a telecentric condition be satisfied at the image side (i.e., rays must be parallel to the optical axis at the image side) in the optical designing of the lens system, thus imposing substantially constraints on design freedom.
On the other hand, FIG. 9B illustrates a case where a bright-dark pattern having a pitch close to the pitch Λ of the pixels 16 (the pitch of the pattern being 10Λ/9 in the illustrated example) is projected onto the photodetector 4. The bright-dark pattern has dark lines 13 which lie upon the pixels 16 at the right and left ends of FIG. 9B, and lie between pixels 16 in the central portion of FIG. 9B. As a result, a pattern which appears bright near the center and dark at the right and left ends of FIG. 9B will be detected as a pseudo signal, the pseudo signal having a large period which is quite different from that of the actual bright-dark pattern.
This stripe pattern is generally known as moire fringes. Since there are four cycling directions of the pixel 16, i.e., the vertical direction, the horizontal direction, and two oblique directions, moire fringes may be caused by a dark pattern which cycles in any of these four directions. In order to prevent moire fringes, the performance of the optical system is intentionally deteriorated with respect to its frequency characteristics in conventional examples (thus lowering the resolution of the optical system, as it were), so that any bright-dark pattern having a pitch close to the pitch Λ of the pixels 16 will be blurred. In other words, the optical system combination is designed to result in a resolution which is lower than the resolution as determined by the number of pixels based on the photosensitive portions 6. As a method for lowering the resolution of the optical system, design parameters may be accordingly adjusted, or alternatively, a birefringent substrate may be placed in a slanted position in front of the photodetector 4 so as to cause blurring of light via birefringence effects. However, when this situation is viewed from the photodetector side, it would appear that a photodetector 4 having a resolution which is unnecessarily higher than the resolution of the optical system is being employed; thus, together with the addition of the birefringent substrate, the overall cost is being increased.
Moreover, a compact imaging device for a mobile phone is required to be thin, which makes it necessary that the focal length f be small. As f decreases, the size of the image 3 (length of each side) also decreases proportionally; as a result, given that the size of the photosensitive portions 6 remains the same, the number of pixels to be utilized for the detection of the image 3 is reduced, whereby the resolution of the image 3 is degraded. Therefore, in order to maintain a good resolution, the size of the photosensitive portions 6 (length of each side) must also be decreased in proportion with the focal length f.
On the other hand, if f is decreased while keeping the same aperture D, it becomes difficult to conserve the optical performance of the lens system 2. In particular, the off-axis characteristics (i.e., optical characteristics with respect to light which enters with an angle from the lens center axis) are deteriorated. In other words, a large aberration is likely to occur in the higher portion of the image 3 (i.e., a region away from the center axis of the lens system 2; that is, a region away from the center of the photodetector 4), so that the resolution in this region is considerably degraded.
Therefore, in order to maintain a certain optical performance, it is necessary to reduce the aperture D at the same rate as f (thus keeping a constant (so-called) “F value”). In the case where the focal length f, the size of the photosensitive portions 6, and the aperture D are all reduced at the same rate, the light intensity of the image 3 will remain unchanged because the F value stays constant; however, since the size of the photosensitive portions 6 is reduced, the light-receiving areas become smaller, so that the light amount detected by each single photosensitive portion 6 becomes smaller. Thus, when the focal length f is reduced for the pursuit of thinness, it may be either that the resolution of the image 3 will be degraded (in the case where the size of the photosensitive portions 6 remains the same); the optical performance will be deteriorated (in the case where the size of the aperture D remains the same); or even in the case where such performance aspects are all conserved, the light amount detected by each photosensitive portion 6 will still be reduced.